1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to communication receivers employing Code Division Multiple Access (CDMA).
2. Description of Related Art
Data communication systems have been under continual development for many years. One particular type of communication system, a cable modem (CM) communication system, has been under continual development for the last several years. There has been development to try to provide for improvements in the manner in which communications between the CM users and a cable modem termination system (CMTS) is performed. Many of these prior art approaches seek to perform and provide broadband network access to a number of CM users.
CM communication systems are realized when a cable company offers network access, oftentimes Internet access, over the cable. This way, the Internet information can use the same cables because the CM communication system puts downstream data, sent from the Internet to an individual computer having CM functionality, into a communication channel having a 6 MHz capacity. The reverse transmission is typically referred to as upstream data, information sent from an individual back to the Internet, and this typically requires even less of the cable's bandwidth. Some estimates say only 2 MHz are required for the upstream data transmission, since the assumption is that most people download far more information than they upload.
Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and the CMTS at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place. This intervening region may be referred to as a CM network segment, and a variety of problems can occur to signals sent across this CM network segment.
One particular deficiency that may arise in this CM network segment is the introduction of multi-path effects where there is interference from one symbol to another in a delayed, scaled form. For example, a scaled and delayed version of one symbol is undesirably added to other symbols. This can lead to significant degradation in performance. In CDMA systems, these multi-path effects can be totally deficient, in that, it may make accurate decoding of the transmitted data virtually impossible. A number of sources may create these multi-path effects, including the communication channel itself, as well as notch filters and interference canceling filters within a communication receiver that may seek to minimize the deleterious effects of a communication channel.
In synchronous code division multiple access (S-CDMA) systems, several cable modems (CMs) transmit their signals such that these signals are received at the CMTS on the same frequency and at the same time. In order for different CM signals to be separated at the CMTS, each CM spreads its data sequence by a code sequence of wider spectrum. The CMTS receives the sum of all CM signals. To separate a specific CM signal, the CMTS despreads the received sequence by multiplying it with the code sequence of the desired CM.
In order to minimize the inter-code-interference (ICI), the spreading codes are chosen such that they are perfectly orthogonal, when they are received in perfect synchronism. In order to guarantee code orthogonality, the code sequences are often chosen to have cyclic-shift properties. To preserve the code orthogonality at the CMTS, transmit equalizers are used by CMs to guarantee a perfect single-path overall channel seen at the CMTS. The transmit equalizer taps at a specific CM are usually set according to an estimate of the channel between the CM and CMTS, which is estimated during the ranging process.
In many cases, the received signal at the CMTS is corrupted with strong narrow-band interference (or ingress). An interference canceling filter (ICF) is used to notch out ingress before signal despreading. Although this ICF can mitigate ingress, it can cause considerable performance degradation, which can be explained as follows. The ICF taps cause inter-symbol-interference (ISI) as they can be viewed as echoes to the signal. These echoes result in shifted (or delayed) replica of the received signal at the CMTS side. This enhances ICI as codes loose their perfect orthogonality. Moreover, due to the cyclic-shift properties of the used orthogonal codes, a shifted replica of one code might resemble another code to a great extent, which enhances the ICI significantly. The same effect can also be caused by imperfections in the channel estimation ranging process, possible channel variations, as well as the finite length and precision of the transmit equalizers, all of which can result in residual echoes in the overall channel seen at the CMTS. The problems described herein may arise within a variety of contexts, including both wireless and wired communication systems.
The effect of ISI in CDMA systems can be extremely problematic and is typically very severe in its magnitude and nature. Because the neighboring codes are typically orthogonal to one another, the ISI look very similar to delayed (shifted and scaled) versions of one another. This interference can be highly correlated and very problematic.
Within Time Division Multiple Access (TDMA) communication systems, a common approach to deal with ISI is to employ a Decision Feedback Equalizer (DFE) type structure. This DFE structure can compensate for interference that takes the form described above that is delayed by at least one symbol. For example, before decoding a second symbol in TDMA using DFE, a scaled version of the first symbol is subtracted there from. The scaling is based on the determined characterization of the ISI that is attributed to the second symbol from the first symbol.
Within the CDMA context, there is no such way known in the prior art to deal with these effects of multi-path and ISI is a satisfactory way. In the CDMA context, a signal is spread out over a number of chips. Here, the ISI will vary on a chip by chip basis. When narrowband interference is undesirably added to these various chips, it makes the decoding of the signal nearly impossible. This is because all of the chips need to be received to perform the soft decisions that are used later to make hard decisions there from. For example, in an embodiment where N chips are used to perform symbol decisions, then all of the 128 chips are needed to N chips need to be received before making the symbol decisions of the symbols contained and coded therein. The interference between these chips will be non-causal in the CDMA context. The very manner in which a CDMA receiver performs decoding of its signals is what makes it impossible to use a DFE type structure (as in TDMA) to perform the decoding. Within CDMA, a clean representation of all of the chips must be achieved. The very way that a CDMA signal is received will inherently include (in the existence of ISI on the chip level) of one clean chip with the remaining chips having the ISI. Since CDMA does not decode a single chip at a time (whereas TDMA may decode a single symbol at a time), this need to decode all of the chips together over a relatively long period of time, for many codes, and sum over all of those codes to decode each symbol. Basically, the fact that, in CDMA, many symbols are all decoded at the same time, there is a need for a clean representation of all of the chips of a received signal.